The monitoring of mechanical strain in structural members can give advanced warning of failure. Such monitoring is generally performed by strain gauges, which also find application in research and development, maintenance and sometimes failure analysis. In the elastic deformation regime, strain gauges can be used to monitor the elasticity of materials. In the plastic regime, strain gauges can be used for monitoring creep and other phenomena.
A number of strain gauges are commercially available and several others have been proposed over the years. Although strain gauges may be based on widely different physical phenomena, essentially, all strain gauges share a common characteristic in that when undergoing a physical deformation, a measurable change in some easily monitored property is caused. To be useful, very small physical changes are required to produce easily measurable changes in some electrical property such as the electrical resistance. To eliminate aberrations and electrical noise, the change should be orders of magnitude more than changes caused by other factors such as changes in temperature, humidity and the like. Strain gauges are useful to give qualitative information, but require calibration to extract quantifiable information.
Sometimes strain gauges are built into mechanical structures to allow continuous or periodic monitoring. Alternatively, strain gauges are retrofitted to structures by being adhered thereto for testing purposes. Sometimes, strain sensing probes are stuck onto the component to be monitored during a testing session and then removed for subsequent use elsewhere.
Strain gauges may be used for a variety of applications such as monitoring strain in bridges and other buildings, on metal structures such as ships and airplanes and for measuring composite materials; both in research and development and for testing during the lifespan thereof. Strain gauges also find application in the seats of vehicles to detect occupancy for warning regarding seatbelt fastening and airbag activation systems.
Strain gauges utilize some physical phenomenon which results in a significant change of an easily measured property for example, electrical resistance. For example, the resistance of a metallic element varies with its dimensions. If a wire is stretched, the cross section perpendicular to the direction of stretching is decreased and the wire becomes elongated. If resistivity is constant, the change in geometry increases the resistance of the component.
To develop precise and sensitive strain gauges, one is required to monitor a constantly changing physical property that varies measurably with strain, but is negligibly affected by other causes.
Preferably, strain gauges provide instantaneous responses that are easily monitored to give real time data. As will be appreciated, the main design criteria are high sensitivity and reproducibility in the desired strain regime for low cost. Usefully, a strain gauge will be able to monitor both tensile and compressive strains. Some strain gauges are designed to monitor bending strains and torques. By monitoring strains whilst applying varying frequency vibrations and thermal cycling, the service degradation of a material or component can be tracked. Sometimes strain gauges are required to follow high frequency dynamic forces to monitor the effects of vibrations, fatigue, creep, crack propagation and the like. Ideally, strain gauges are stable and give reproducible results over long time periods with negligible temperature drift, large outputting and high noise-resistance.
One known physical property that may be used in strain gauges is the permeability of ferromagnetic materials which assume regions of uniform magnetic polarization known as Weiss domains. In some ferromagnetic materials, the Weiss domains tend to align when the material is strained. This phenomenon is known as the Villari or “reverse magnetostrictive” effect and materials exhibiting the effect are known as magnetostrictive materials. Thus, essentially, a magnetostrictive core exhibiting the Villari effect has magnetic properties that are alterable by application of a strain. A small physical deformation causes a large change in the magnetic properties, particularly the permeability of the magnetostrictive material, and these changes are easily monitored.
For example, U.S. Pat. No. 3,638,153 to Sparrow (Honeywell), the disclosure of which is incorporated herein by reference, describes a transducer having a single layered magnetostrictive member.
U.S. Pat. No. 4,697,460 to Sugiyama et al. (Toyota) titled “Device for measuring torque of a rotary mechanism” describes a torque sensor for electrically detecting torque, such as the torque of an automobile engine, wherein a strain disk is provided in the path of torque transmission and the torque is magnetically detected in terms of the magnetostriction generated in the strain disk. Typically, the magnetostrictive layer is formed by sputtering or by vacuum deposition.
Japanese Patent Publication Number JP2004245619 to Sasaki et al. (Yaskawa Electric Corp) describes a magnetostriction-type distortion sensor that uses a resin containing a soft magnetic powder for connecting a solenoid coil to the circumference of a soft magnetic region.
U.S. Pat. No. 4,920,806 (also European Patent Number EP0329479) to Toshiba, describes a strain gauge that includes a pair of coils printed on one face of a substrate and an amorphous magnetic metallic plate having a magnetostrictive effect arranged on the other face of the substrate. Magnetic flux generated by one of the paired coils passes through the amorphous magnetic metallic plate and links with the other coil. When an object is loaded, resulting in a deformation (strain), the magnetic permeability of the amorphous magnetic metallic plate is changed, due to the magnetostrictive effect, in response to the loading. The density of the magnetic flux passing through the magnetic metallic plate is also changed in response to this changing magnetic permeability and the composite inductance of the paired coils changes in response to the changing density of the magnetic flux. The strain gauge outputs a detection signal which represents the changing density of the magnetic flux, and this signal can be calibrated to measure the load added or the strain caused by the load. The detecting sensitivity of the strain gauge is high. It has high reliability and can be handled with ease. Because the coils are arranged side by side within the probe, the flux linkage is only partial and utilization of the magnetostrictive effect is not optimal.
U.S. Pat. No. 5,437,197 to Uras describes a magnetic sensor comprising an exciter coil and a sensing coil linked by a magnetic flux. U.S. Pat. No. 5,449,418 to Tagaki et al. describes a method of forming a magnetostrictive layer and a strain sensor using same, wherein the amorphous, magnetostrictive material is formed by a high energy beam, such as a laser, for example.
U.S. Pat. No. 5,493,220 to Oliver (Northeastern University) describes a magneto optic stress sensors using the Kerr effect.
U.S. Pat. No. 5,675,252 to Podney (SQM) describes a magnetic and electric field sensor and measurement apparatus comprising a composite structure having alternating secured plural piezoelectric material layers and magnetostrictive material layers, magnets positioned adjacent to the composite structure for supplying a bias field to the composite structure, and a measurement circuit connected to the piezoelectric layers for measuring output of the piezoelectric layers in the composite structure. U.S. Pat. No. 6,622,577 to Uras describes a single coil magnetostrictive force or strain sensor.
U.S. Pat. No. 7,093,499 to Baudendistel (Delphi Technologies) entitled “Force sensor, strain sensor and methods for measuring same” relates to an inductance measuring assembly connected to a coil for measuring the inductance in the coil wherein the measured inductance of the coil changes equally with addition of equal point loads. Specifically, there is described a force sensor comprising: a coil adapted to carry an electric current; a quantum tunneling composite electrically insulated from the coil, disposed in a magnetic path created by the coil when an alternating current is present in the coil, and disposable in a load path of a force to be under strain from the force; and an inductance measuring assembly operatively connected to the coil to measure an inductance in the coil when the alternating current is present in the coil and when the quantum tunneling composite is disposed in the load path of the force to be under strain from the force, wherein the force sensor determines the force using at least the measured inductance in the coil.
U.S. Pat. No. 7,146,866 to Morelli (Delphi Technologies), titled “Magnetostrictive strain sensor and method” describes a magnetostrictive sensor for determining an applied strain that has an in-phase voltage circuit which senses in-phase voltage that is in-phase with an alternating current and which varies correspondingly to a strain on the magnetostrictive core. The magnetostrictive core comprises a magnetostrictive material, such as a nickel-iron alloy, which is able to conduct a magnetic flux and whose permeability is alterable by application of a strain. The suggested material is 50-70% nickel and 30-50% iron. However, other magnetostrictive materials are also suitable, including, but not limited to, nickel, iron, rare earth-iron alloys, and other magnetic materials that exhibit appreciable magnetostrictive coefficients.
A variety of magnetostrictive materials have been employed in sensors. Many of them are deposited by plating techniques. For example: Japanese Patent Number JP632020 titled “Amorphous Magnetic Material By Plating Method” describes an amorphous iron-phosphorus alloy containing 8-23 atomic % phosphorus, deposited as thin films by plating techniques. Similarly, European Patent Number EP0447044 titled “Magneto-elastic film and process” describes a cobalt-iron alloy magneto-elastic film having from about 20 to about 40 atomic weight iron. U.S. Pat. No. 5,194,806 to Obama describes a sensor arrangement that includes a magnetostrictive film of, for example, Co 85% Zr 10% Fe 5%.
Another technique for obtaining an amorphous metal layer is by local melting using a laser, where fast cooling results in the formation of a amorphous metal layer; see for example: U.S. Pat. No. 5,449,418 to Takagi, (Nippondenso Co., Ltd.) titled “Method of formation of magnetostrictive layer and strain sensor using same” which describes a strain sensor for detecting strain in automobiles, robots, and the like, the formation of the magnetostrictive film of which is by forming an alloy film and scanning with overlapping a high energy density beam to form a magnetostrictive layer having an amorphous structure, wherein the resulting magnetostrictive layer has a magnetic inductance in the direction of irradiation that is much higher than in a perpendicular direction thereof. Using such a sensor, it is possible to selectively detect components of stresses and strains in a given direction.
U.S. Pat. No. 5,347,872 titled “Magnetomechanical sensor attachment method” describes a sensor fabricated by bonding a metallic glass ribbon to an object to be measured using a homogeneous viscous organic liquid. A suggested material for the metallic glass ribbon is FewBxSiyCz having the composition: 0.70≦w≦0.83, 0.10≦x≦0.20, 0.03≦y≦0.10, 0≦z≦0.03, and w+x+y+z=1.00
U.S. Pat. No. 4,938,267 titled “Glassy metal alloys with perminvar characteristics” describes an amorphous cobalt based magnetic alloy—with near zero magnetostriction and perminvar characteristics The glassy alloys have the compositions CoaFebNicMdBeSif where M is at least one member selected from the group consisting of Cr, Mo, Mn and Nb, and “a-f” are in atom percent where “a” ranges from about 66 to 71, “b” ranges from about 2.5 to 4.5, “c” ranges from about 0 to 3, “d” ranges from about 0 to 2 except when M=Mn in which case “d” ranges from about 0 to 4, “e” ranges from about 6 to 24 and “f” ranges from about 0 to 19, with the proviso that the sum of “a”, “b” and “c” ranges from about 72 to 76 and the sum of “e” and “f” ranges from about 25 to 27.
U.S. Pat. No. 4,763,030 titled “Magnetomechanical energy conversion” describes a magneto-structure transducer that uses a ribbon element of iron-boron-silicon-carbon metallic glass of the formula FewBxSiyCz wherein 0.78≦w≦0.83, 0.13≦x≦0.17, 0.03≦y≦0.07, 0.005≦z≦0.03 and w+x+y+z=1. The ribbon is annealed to remove mechanical strains and is then exposed to a magnetic field in the plane of the ribbon and transverse to the long axis of the ribbon. The resulting metallic glass ribbons have very large magnetic coupling coefficients. The treated ribbons are useful in magnetostrictive transducers and in passive listening devices such as hydrophones or pressure sensors.
U.S. Pat. No. 6,639,402 titled “Temperature, stress, and corrosive sensing apparatus utilizing harmonic response of magnetically soft sensor element(s)” to Grimes, relates to a temperature sensing apparatus including a sensor element made of a magnetically soft material operatively arranged within a first and second time-varying interrogation magnetic field, the first time-varying magnetic field being generated at a frequency higher than that for the second magnetic field. A receiver, remote from the sensor element, is engaged to measure the intensity of electromagnetic emissions from the sensor element to identify a relative maximum amplitude value for each of a plurality of higher-order harmonic frequency amplitudes thus measured. A unit then determines a value for temperature (or other parameter of interest) using the relative maximum harmonic amplitude values identified. In other aspects of the invention, the focus is on an apparatus and technique for determining the stress condition value of a solid analyte and for determining a value for corrosion, using the relative maximum harmonic amplitude values identified. A magnetically hard element supporting a biasing field adjacent the magnetically soft sensor element can be included. It will be appreciated however, that the technique of measuring harmonics does not give high sensitivity. Measuring second and higher order harmonics is unduly complicated and sometimes an irreversible response is caused by application of a relatively small load. Of interest, the magnetostrictive materials used were the iron-rich Fe81B13.5Si3.5C2 (METGLAS® 2605SC) and Fe40Ni38Mo4B18 (METGLAS® 2826MB). Various techniques have been suggested for enhancing the response of strain sensors using magnetostrictive alloys. For example, Japanese Patent Application Number JP 61240132 entitled “Sensor Detection Method” addresses the issue of enhancing the reproducibility, sensitivity and stability in detecting the inductance value of a sensor constituted of an amorphous alloy having magnetostriction, by repeatedly using an AC signal having constant voltage continuously changing from low frequency to high frequency wherein the electric current applied is an alternating current superposed with a cyclically recurring rectangular current which has, in each cycle thereof, a large amplitude at a start-up point and which converges to a smaller amplitude after a predetermined time period. This and other techniques aim to get improved sensitivity and reliability, or, on other words, a large signal to noise ratio.
There is still a need for cheap, reliable, highly sensitive strain gauges and methods of measuring strain and the present invention addresses these needs.